Magnetic resonance tomography (MRT) is an imaging method which enables the recording of two-dimensional (2D) or three-dimensional (3D) image data sets which can image structures inside a person being examined, in particular also soft tissue, with high resolution. In the case of MRT the magnetic moments of protons in an object to be examined are oriented in a basic magnetic field. The nuclear spins can be deflected or excited from the oriented position, i.e. the rest position, or a different state by radiating high frequency pulses. The evolution over time of the excited magnetization is then detected by way of one or more high frequency (HF) coil(s).
By applying a layer selection gradient when radiating the high frequency pulses only nuclear spins in which the resonance condition is fulfilled owing to the local magnetic field strength are excited in a layer of the object to be examined. Further spatial encoding can occur by applying at least one phase encoding gradient and a frequency encoding gradient during reading-out or a signal detection period. It is consequently possible to obtain MR images of a plurality of layers of a person being examined. By means of suitable display methods it is thus possible to provide for diagnosis a 3-dimensional (3D) image of a certain region of the person being examined in the form of a 3D MR image.
However, the measureable volume is typically limited in a magnetic resonance (MR) system owing to physical and technical conditions, such as a limited homogeneity of the basic magnetic field and a non-linearity of the gradient field. A measuring range, what is known as a “Field of View” (FoV), is therefore limited to a volume in which the physical features mentioned above lie within a predefined tolerance range and therefore imaging, which is true to the original, of the object to be examined is possible with conventional measuring sequences. The Field of View limited in this way is, in particular in the x and y directions, i.e. perpendicular to a longitudinal axis of a ring tunnel of the magnetic resonance system, considerably smaller than the volume limited by the rung tunnel of the magnetic resonance system. In conventional magnetic resonance systems a diameter of the rung tunnel is by way of example about 60 cm, whereas the diameter of the conventionally used Field of View, in which the physical features mentioned above lie within the tolerance range, is for example 50 cm.
Significant differences in the homogeneity of the basic magnetic field can occur outside of the Field of View. An MR image which has been taken exhibits distortions in the corresponding region. There are various applications in which a high level of faithfulness to location, i.e. low distortion of the MR image data, is necessary.
In the case of hybrid systems for instance, such as a hybrid system comprising a magnetic resonance tomograph and a positron emission tomograph, what is known as an MR/PET hybrid system, it is very important to determine structures of the object to be examined as accurately as possible, even in the edge region. With an MR/PET hybrid system the human attenuation correction by way of example is of crucial importance. With the human attenuation correction the intensity attenuation of the photons emitted after an interaction of positrons and electrons is determined on their path through absorbent tissue to the detector and the received signal of the PET corrected by precisely this attenuation. MR data is acquired for this purpose which images the complete anatomy of the object to be examined in the direction of the high-energy photons emitted by the positron emission tomography. The anatomy of the object to be examined should therefore be detected as accurately as possible even in the edge region of the tunnel of the hybrid system. Structures which are located in these regions are, in the case of patients to be examined, by way of example the arms, which can be arranged in the edge region close to the tunnel inside wall of the hybrid system.
Further applications in which a high level of faithfulness to location is required are MR-based interventions, MR-based radiation planning for radiation therapy systems, whole body MR applications, such as in MR oncology and MR angiography in particular when used on short magnets which have a particularly limited FoV, and post-processing applications such as “composing” or “fusion” with imaging methods with a high level of faithfulness to location, such as computed tomography or PET.
To expand the Field of View methods are known in the literature for example which enable the non-linearity of a gradient field used for spatial encoding during the recording of MR data to be used to compensate an inhomogeneity of the basic magnetic field. To record 3D MR images it is necessary to record MR data from a plurality of layers. However, since the inhomogeneity of the basic magnetic field is location-dependent, compensation at a plurality of sites is complicated or possible only with difficulty using the known methods. Gaps can result for example between the individual layers, via which no MR information may be obtained and which therefore have to be interpolated. This limits the possibility of producing expanded Field of View 3D MR images.